1. Field of the Invention
Various embodiments of the invention are directed to logging tools, such as wireline tools and logging tools used while drilling. More particularly, various embodiments of the invention are directed to calibration of sensors to compensate for tool drift which may be associated with temperature and/or age of the tool.
2. Description of the Related Art
Modern drilling operations demand a great quantity of information relating to the parameters and conditions encountered downhole. Such information typically includes characteristics of the earth formations traversed by the wellbore, as well as information regarding the wellbore itself.
The collection of information relating to conditions downhole, which is commonly referred to as “logging,” may be performed by several methods. In wireline logging, a probe or “sonde” is suspended in the borehole by way of an armored cable (the wireline) after some or all of the well has been drilled. There are also tools that collect data during the drilling process. By collecting, processing and transmitting data to the surface real-time while drilling, the timeliness of measurement data of formation properties is improved and, consequently, the efficiency of drilling operations is increased. Tools that are used while drilling may be referred to as measurement-while-drilling (MWD) or logging-while-drilling tools (LWD). While there may be some distinction between MWD and LWD, the terms are often used interchangeably, and for purposes of this specification the term LWD will be used with the understanding that LWD may also refer to MWD operations.
A formation containing hydrocarbons has certain well known physical characteristics, such as resistivity (the inverse of conductivity) within a particular range. Measurements of resistivity are based on attenuation and phase shift of an electromagnetic signals propagating through the formation, and thus it is important to measure amplitude and phase shift accurately. Even small amounts of error are relatively significant given the small amplitude of signals detected at the receiver, which are often on the order of 10 nV. A long-standing phenomenon known as tool drift introduces errors in the measurement of attenuation and phase shift. In particular, as tool temperature varies, and as the tool ages, measurements of attenuation and phase shift of a received electromagnetic signal drift. The amount of drift varies from tool to tool, and can be substantial in deep wells where temperatures can exceed 150° Celsius.
In order to compensate for tool drift, related art logging tools may have their response as a function of temperature determined prior to deployment into the borehole. The downhole measurements are then compensated based on downhole temperature and the temperature response characteristics of the tool. However, determining the temperature response characteristics of a tool is a very time consuming and labor intensive process, and does not account for other drifts that may be encountered in a logging tool, such as the effect of aging. Other techniques may be to use a “compensated” logging tool having multiple symmetric receiver pairs. However, tools that use multiple symmetric receiver pairs require additional components and processing. Compensated tools tend to be longer, thus increasing cost. Moreover, “compensated” tool design requires a particular physical structure of the tool, and thus older tools may not be suited to be retrofitted with multiple symmetric receiver pairs.